There has recently been proposed a use of a high pressure discharge lamp as a light source of a liquid crystal projector, an automobile headlight or the like. As shown in FIG. 21, a discharge lamp lighting device for lighting this kind of high pressure discharge lamp is typically constituted such that: a voltage of a direct current power source (including a pulsating power source obtained by full-wave rectifying a commercial power source) E is stepped down by a step down type chopper circuit 1; an output voltage of the chopper circuit 1 is smoothed by a smoothing capacitor C1; a direct current voltage as a voltage across the smoothing capacitor C1 is converted into an alternating voltage whose polarity is to be alternated by a polarity inversion circuit 2 which comprises a full bridge circuit; and the alternating voltage outputted from the polarity inversion circuit 2 is applied to a load circuit including a high pressure discharge lamp La. The load circuit comprises a filter circuit consisting of a series circuit of a capacitor C2 and an inductor L2, and has a constitution where the high pressure discharge lamp La is connected in parallel with the capacitor C2. That is, a rectangular wave voltage from which a high frequency element has been removed by the filer circuit is applied to the high pressure discharge lamp La.
The chopper circuit 1 has a serial circuit of a switching element Q1 made of a metal-oxide semiconductor field-effect transistor (MOSFET) and an inductor L1, which have been inserted between the direct current power source E and the smoothing capacitor C1, and a diode D1 is connected in parallel with the serial circuit of the inductor L1 and the smoothing capacitor C1. The polarity of the diode D1 is determined such that energy which is stored in the inductor L1 when the switching element Q1 is ON is then discharged as a regeneration current through the smoothing capacitor C1 when the switching element Q1 is OFF. Further, in the illustrated example, a resistor R1 for detecting a current is inserted between the negative electrode of the direct current power source E and the anode of the diode D1. The terminal voltage of the smoothing capacitor C1 is parted by a voltage detecting circuit 3 consisting of a serial circuit of two resistors R2 and R3, and a voltage across the resistor R3 is outputted, as a voltage proportional to the terminal voltage of the smoothing capacitor C1, from the voltage detecting circuit 3.
A polarity inversion circuit 2 is a circuit where four switching elements Q2 to Q5 each made of a MOSET are bridge-connected, and a serial circuit of the switching elements Q2 and Q3 and a serial circuit of the switching elements Q4 and Q5 are each connected as an arm of the bridge circuit between each terminal of the smoothing capacitor C1. A load circuit is connected between a connection point of the switching elements Q2 and Q3 and a connection point of the switching elements Q4 and Q5. That is, a state where the switching elements Q2 and Q5 are on while the switching elements Q3 and Q4 are off and a state where the switching elements Q2 and Q5 are off while the switching elements Q3 and Q4 are on are controlled so as to be alternately repeated, whereby an alternating voltage is applied to the load circuit. Since the load circuit includes the serial circuit of the capacitor C2 and the inductor L2, and a voltage across the capacitor C2 is applied to the high pressure discharge lamp La, the lamp current of the high pressure discharge lamp La can be changed by changing a frequency (hereinafter referred to as “inversion frequency”) for on/off of the switching elements Q2 to Q5.
The on/off of the switching elements Q1 to Q5 included in the chopper circuit 1 and the polarity inversion circuit 2 are controlled by a control circuit 4. The control circuit 4 starts controlling the switching elements Q1 to Q5 in the chopper circuit 1 and the polarity inversion circuit 2 when a lightning signal is inputted from an exterior portion, and the control circuit 4 changes an output power of the chopper circuit 1 when an electric power switching signal S2 is inputted from an external portion. Further, the control circuit 4 monitors, with a voltage across the resistor R1, a current corresponding to the lamp current of the high pressure discharge lamp La, and also monitors an output voltage of the voltage detecting circuit 3, to perform pulse-width-modulation (PWM) control of the switching element Q1 of the chopper circuit 1 so as to maintain electric power instructed by the electric power switching signal S2. Moreover, the control circuit 4 outputs a control signal for turning the switching elements Q2 to Q5 on and off, and the control signal is provided to the switching elements Q2 to Q5 through drivers 2a and 2b. An on/off duty ratio of the switching elements Q2 to Q5 is here set to 50% so as to equally wear out two electrodes disposed in the high pressure discharge lamp La.
Incidentally, the high pressure discharge lamp La for use as a liquid crystal projector or an automobile headlight has electrodes dose to one another and can thus be used as a point source, and it is known that, in this kind of high pressure discharge lamp La, a phenomenon occurs where a luminescent spot on the electrode, i.e. a radiant point of an electron current when the electrode is on the cathode side, is not stabilized in a fixed position and moves disorderly. This phenomenon is called an arc jump, and when the arc jump occurs in a light source for a liquid crystal projector, a luminescent spot is displaced with respect to an optical system to be used along with the light source, causing a problem of variations in light amount on a screen. That is, a change in electric power to be charged during lightening of the high pressure discharge lamp La leads to variations in temperature of or distance between the electrodes, and further when a fan for air cooling is built in a housing like a liquid crystal projector, a change in condition for air cooling leads to variations in temperature of or distance between the electrodes. As thus described, when the state of the electrodes varies, a voltage across the electrodes varies, resulting in occurrence of an arc jump. Especially when the illuminating time of the high pressure discharge lamp La becomes longer, the voltage across the electrodes increases, and also when supply power to the high pressure discharge lamp La is switched in the lower electric power direction, the lamp current decreases to cause lowering of the electrode temperature, thereby making the arc jump tend to occur.
In a state where the high pressure discharge lamp La is stably on, the lamp current varies as the voltage across the smoothing capacitor C1 is changed by PWM controlling the switching element Q1 of the chopper circuit 1. That is, the lamp current varies by changing either the on/off duty ratio of the switching element Q1 of the chopper circuit 1 or the inversion frequency of the switching elements Q2 to Q5 of the polarity inversion circuit 2. However, a knowledge has been obtained that there exists a relation for stabilizing the state of the electrodes of the high pressure discharge lamp La, between the voltage across the smoothing capacitor C1 (which corresponds to the lamp voltage, as described later) and the frequency of the alternating voltage to be applied to the high pressure discharge lamp La. In other words, it has been found that there exists an optimum value of the inversion frequency according to the lamp voltage (the voltage across the smoothing capacitor C1) to the polarity inversion circuit 2, as a condition for reducing variations in temperature of or distance between the electrodes to keep the electrodes in a stable state. Therefore, if the inversion frequency and the lamp voltage of the polarity inversion circuit 2 in combination are optimum values, the occurrence of the arc jump is suppressed to reduce the wearing out of the electrodes, thereby extending the life of the high pressure discharge lamp La.
In the following, the relation between the lamp voltage and the inversion frequency in the polarity inversion circuit 1 is considered. Firstly considered is the case where the inversion frequency is controlled so as to be kept constant irrespective of the lamp voltage. The optimum value of the inversion frequency is here set to f1 in the range of lamp voltages from V1 to V2. When the inversion frequency is controlled so as to be kept at f1 irrespective of the lamp voltage as shown by A in FIG. 22, the inversion frequency f1 is the optimum value in the range of lamp voltages from V1 to V2 as shown by B1, whereas the optimum value of the inversion frequency is f2 in the range of lamp voltages lower than V1 as shown by B2, and the optimum value of the inversion frequency is f3 in the range of lamp voltages higher than V2 as shown by B3, indicating that the inversion frequency is not the optimum value in either range of lamp voltages. That is, when the inversion frequency is fixed, in the range of the lamp voltages from V1 to V2, the state of the electrodes of the high pressure discharge lamp La is stabilized, allowing inhibition of the occurrence of the arc jump, whereas, when the lamp voltage is lower than V1 or higher than V2, the inversion frequency is deviated from the optimum value and the state of the electrodes of the high pressure discharge lamp La thus become unstable, leading to occurrence of the arc jump.
Next, considered is the case where the electric power switching signal S2 instructs switching of the electric power and the inversion frequency of the polarity inversion circuit 2 is controlled so as to be kept constant irrespective of the instructed electric power. As shown by A in FIG. 23, the optimum value of the inversion frequency is here set to f1 in the range of lamp voltages from V1 to V2 when an electric power is P1. When the electric power is switched from P1 to P2, the lamp voltage of the polarity inversion circuit 2 varies and the lamp voltage of the high pressure discharge lamp La then varies to cause deviation of the electrodes of the high pressure discharge lamp La from the stable state, leading to the shift of the optimum value of the inversion frequency to the frequency f2 as shown by B in FIG. 23. However, since the inversion frequency is here controlled so as to be kept constant irrespective of the electric power, the electrodes consequently become unstable to lead to occurrence of the arc jump.
As another example for the control, as shown in FIG. 24, there can also be considered a method of continuously changing the inversion frequency of the polarity inversion circuit 2 according to the lamp voltage. In the illustrated example, the inversion frequency is f1 when the lamp voltage is V1, and the inversion frequency is f2 when the lamp voltage is V2. That is, it is considered that, since the lamp voltage is constantly kept at the optimum value at inversion frequencies from f1 to f2 in the range of lamp voltages from V1 to V2, the state of the electrodes is kept stable. However, since even slight variations in lamp voltage are followed by variations in inversion frequency, the duty ratio of the lamp current in the current waveform becomes different from 50% as revealed from FIG. 25(a), which may raise a problem of unequal wearing out of the electrodes to thereby shorten the life of the high pressure discharge lamp La.
In order to solve this kind of problem, a constitution has been proposed where information corresponding to the distance between the electrodes is monitored by the lamp voltage, the inversion frequency is made switchable in two stages, a width of increase/decrease of the lamp voltage from an initial value is detected, and the inversion frequency is increased when the lamp voltage is on the decrease and the increase/decrease width is larger than a prescribed threshold, while the inversion frequency is decreased when the lamp voltage stops increasing/decreasing (see e.g. Patent Document 1: Japanese Patent No. 3327895, p 10-11, FIG. 7)